1. Field of the Invention
The present invention relates to an electric rotating machine such as an alternator (generator) mounted on a passenger car or a truck.
2. Description of Related Art
In recent years, there has been an increasing demand for a vehicle alternators driven by an engine mounted on a vehicle, which generate less magnetic noise at the time of power generation, in order to contribute to upgrading the vehicle. However, on the other hand there has been an increasing demand for vehicle alternators having high power generation capacity because of an increase of electrical load due to an increased number of safety control devices and the like mounted on the vehicle. Accordingly, it has been required to provide small-sized, low-noise, and high-output vehicle alternators at a lower cost.
To address such requirements, various methods for reducing magnetic noise of a vehicle alternator have been proposed. For example, Japanese Patent Application Laid-open No. 2002-247787 discloses an electric rotating machine with a stator including a stator core and an armature winding wound around the stator core. This electric rotating machine has a structure in which the armature winding includes two sets of three-phase windings each of which is delta-connected (Δ-connected), and each output terminal of one of the three-phase windings is series-connected to a corresponding phase coil of the other of the three-phase windings. According to this structure in which the delta-connected three-phase winding (may be referred to as delta winding hereinafter) is series-connected with the phase coils of the star-connected (Y-connected) three-phase winding (may be referred to as star winding hereinafter), it is possible to prevent a circulation current from flowing even when the generation voltages of theses three-phase windings are different from each other. This makes it possible to implement a small-sized and highly efficient alternator because there is no output loss due to circulation current. This patent document further discloses an electric rotating machine having a structure as shown in FIG. 9, in which two sets of armature windings are arranged so as to form an electrical angle of π/6 therebetween, each of the two armature windings being constituted by a delta winding and a star winding.
According to this structure in which the two three-phase windings are wound around a stator core with a phase difference of π/6 in electrical angle therebetween, the high frequency components of the reactive magnetomotive forces by the two three-phase windings, which are the main cause of magnetic noise, cancel out each other to thereby reduce the magnetic noise. However, since this structure needs two sets of rectifier devices for converting an AC current induced in the stator into a DC current, the manufacturing cost thereof is high. In addition, as shown in FIG. 10, in this structure, since the delta winding and star windings are located in different slots, the effect of reduction of magnetic noise is lowered due to magnetic force imbalance.